Bookmarks / multiple accounts
How do I merge multiple Firefox accounts into one account?
Are you talking about multiple sync accounts? They can not be merged.
If you are talking about multiple profiles, very simple. Log each profile one at a time into one sync account. Give the profile time to do the sync. The time varies.
After the first one is done, you can then sync the next profile. Then the next.
Note: You can not sync all data between different Firefox versions or different computer systems because some things, like add-ons and some settings, are not compatible.
You can use sync to share passwords, bookmarks, and history. Anything else may not be compatible.